a) If is the radius of a circle then:

- Perimeter
- Area or Area
- Diameter

b) If is the radius of a circle then:

- Perimeter of a semi circle
- Area of semi circle

c) If is the radius of a circle then:

- Perimeter of the quadrant
- Area of the quadrant

d) If and are radii of two concentric circles, then

- Area enclosed between the two circle

e) If is an equilateral triangle of side and height , then .

Also if and are the radii of the circumscribed and inscribed circles of , then and

- Circumference of the circumcircle of
- Area of the circumcircle
- Circumference of the inscribed circle
- Area of the inscribed circle

f) Important notes:

- If two circles touch internally, then the distance between their centers is equal to the difference of their radii.
- If tow circles touch externally. Then the distance between their centers is equal to the sum of their radii.
- The distance moved by a rotating wheel in one revolution is equal to the circumference of the wheel.
- The number of revolutions completed by a rotating wheel in one minute

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